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/Og (Global Optimizations)

/Og

This option provides local and global optimizations, automatic-register allocation, and loop optimization.

  • Local and global common subexpression elimination

    In this optimization, the value of a common subexpression is calculated once. In the following example, if the values of b and c do not change between the three expressions, the compiler can assign the calculation of b + c to a temporary variable, and substitute the variable for b + c:

    a = b + c;
    d = b + c;
    e = b + c;
    

    For local common subexpression optimization, the compiler examines short sections of code for common subexpressions. For global common subexpression optimization, the compiler searches entire functions for common subexpressions.

  • Automatic register allocation

    This optimization allows the compiler to store frequently used variables and subexpressions in registers; the register keyword is ignored.

  • Loop optimization

    This optimization removes invariant subexpressions from the body of a loop. An optimal loop contains only expressions whose values change through each execution of the loop. In the following example, the expression x + y does not change in the loop body:

    i = -100;
    while( i < 0 )
    {
        i += x + y;
    }
    

    After optimization, x + y is calculated once rather than every time the loop is executed:

    i = -100;
    t = x + y;
    while( i < 0 )
    {
        i += t;
    }
    

    Loop optimization is much more effective when the compiler can assume no aliasing, which you set with Assume No Aliasing (/Oa) or Assume Aliasing Across Function Calls (/Ow).

    Note   You can enable or disable global optimization on a function-by-function basis using the optimize pragma with the g option.

For related information, see Generate Intrinsic Functions (/Oi), Improve Float Consistency (/Op), and Full Optimization (/Ox).

To set this compiler option in the Visual Studio development environment

  1. Open the project's Property Pages dialog box. For details, see Setting Visual C++ Project Properties.
  2. Click the C/C++ folder.
  3. Click the Optimization property page.
  4. Modify the Global Optimizations property.

To set this compiler option programmatically

See GlobalOptimizations Property.

See Also

/O Options (Optimize Code) | Compiler Options | Setting Compiler Options

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